ABSTRACT
Our group has recently developed a new method for highly stereoselective glycosylation that was named the hydrogen-bond-mediated aglycone delivery (HAD). 1 We have collected convincing mechanistic evidence that HAD reactions proceed through the formation of a hydrogen bond between a picoloyl or picolinyl substituent on the glycosyl donor and the hydroxyl group of a glycosyl acceptor. As a result, excellent syn-selectivity with respect to the remote picolinyl substituent is achieved. Here, we present a typical glycosylation reaction between 4-O-picoloylated glycosyl donor 2 and the glycosyl acceptor 3. 2 It afforded 1,2-cis-linked disaccharide 4 with complete α-selectivity in good yield. We recently demonstrated that complete selectivity and high yields can also be achieved in the presence of bromine as the promoter. 3 The versatility of the picoloyl group is that it can be selectively removed with Cu(OAc)2 in the presence of other ester protecting groups. Thus, disaccharide 4 could be converted into a glycosyl acceptor of the second generation 5 that is suitable for further modifications. Our previous study showed that this sequence can be reiterated to obtain 1,2-cis-linked glycans up to a pentasaccharide. 4 Other applications of the HAD method include stereoselective syntheses of glycosides of the galacto, 1 rhamno, 1 manno, 5 arabinofurano, 6 2-deoxyglyco, 7 and 2,3-dehydroglyco series. 8
